Fatigue strength of Ti–6Al–4V at very long lives

The objective of this research was to investigate the fatigue strength of Ti–6Al–4V using an ultrasonic fatigue system. Fatigue testing up to 10⁹ cycles under fully reversed loading was performed to determine the ultra-high cycle fatigue behavior of Ti–6Al–4V. Endurance limit results were compared to similar data generated on conventional servohydraulic test systems and electromagnetic shaker systems to determine if there are any frequency effects. Fatigue specimens were tested with and without cooling air to determine the effects of increased specimen temperature caused by internal damping due to cycling at a very high frequency. An infrared camera was also used to record specimen temperatures at various load levels. Results indicate that the effects of frequency, including internal heating, on the very high cycle fatigue behavior of Ti–6Al–4V are negligible under fully reversed loading conditions.     

Effects of stress ratio and temperature on fatigue crack growth in a Ti–6Al–4V alloy

Fatigue thresholds and fatigue crack growth (FCG) rates in corner notched specimens of a forged Ti–6Al–4V aero-engine disk material were investigated at room temperature and 350 °C. The threshold stress intensity range, ΔK_th, was determined by a method involving a step change in stress ratio (the ‘jump in’ method). It was found that for three high stress ratios (R=0.7–0.9), where crack closure effects are widely accepted to be negligible, there were similar ΔK_th values at room temperature and 350 °C under the same R. For a given temperature, ΔK_th was observed to decrease from 3.1 to 2.1 MPam with R increasing from 0.7 to 0.9. The fatigue crack growth rate was influenced by increasing temperature. For high stress ratios, FCG rate at 350 °C was higher than that at room temperature under the same ΔK. For a low stress ratio (R=0.01), higher temperature led to higher FCG rates in the near-threshold regime, but showed almost no effect at higher ΔK. The influence of stress ratio and temperature on threshold and FCG rates was analysed in terms of a K_max effect and the implication of this effect, or related mechanisms, are discussed. In light of this, an equation incorporating the effects of the K_max and fatigue threshold, is proposed to describe FCG rates in the near-threshold and Paris regimes for both temperatures. The predictions compare favourably with experimental data.     

A multiphase mechanical model for Ti–6Al–4V: Application to the modeling of laser assisted processing

A multiphase model for Ti–6Al–4V is proposed. This material is widely used in industrial applications and so needs accurate behaviour modeling. Tests have been performed in the temperature range from 25 °C to 1020 °C and at strain rates between 10⁻³ s⁻¹ and 1 s⁻¹. This allowed the identification of a multiphase mechanical model coupled with a metallurgical model. The behaviour of each phase is calibrated by solving an inverse problem including a phase transformation model and a mechanical model to simulate tests under thermomechanical loadings. A scale transition rule (β-rule) is proposed in order to represent the redistribution of local stresses linked to the heterogeneity of plastic strain. Finally this model is applied to two laser assisted processes: direct laser fabrication and laser welding.     

Variability in room temperature fatigue life of alpha + beta processed Ti–6Al–4V

The variability in fatigue behavior is often what drives the design of components such as turbine engine blades and disks. These components are critical and must be designed with a very low probability of failure over the lifetime of the system. To meet that design criterion, the lower limit of fatigue life capability is typically used. The challenge is to reliably predict the lower limit of fatigue behavior. This study investigates the fatigue variability of an alpha + beta processed Ti–6Al–4V turbine engine alloy by conducting a statistically significant number of repeated tests at a few conditions. Testing includes three conditions including two maximum stresses, 675 and 635 MPa; and two surface conditions, electropolished and low stress grinding. All tests are constant amplitude with a stress ratio of 0.1. A similar approach has been performed on several other turbine engine material systems often revealing a bimodal behavior. It is proposed that crack propagation using small crack growth data can be used to predict the low life behavior mode and is demonstrated with the Ti–6Al–4V data.     

Diffusion bonding between Ti‐6Al‐4V alloy and interstitial free steel

This work deals with diffusion bonded joints between Ti–6Al–4V alloy and interstitial free (IF) steel at different temperatures under a pressure of 5 MPa for 30 min. The effect of bonding temperature on the microstructure and mechanical properties of the joint interface was investigated using optic microscopy, a scanning electron microscope (SEM) equipped with X‐ray energy dispersive spectrometer (EDS) and shear strength measurements. The intermetallic phases such as FeTi and Fe₂Ti occurred at the interface of bonded specimens. In addition, it was seen that shear strength of bonded specimens decreased with increasing temperature due to growing intermetallics.     

Low-temperature compaction of Ti–6Al–4V powder using equal channel angular extrusion with back pressure

Equal channel angular extrusion (ECAE), with simultaneous application of back pressure, has been applied to the consolidation of 10 mm diameter billets of pre-alloyed, hydride–dehydride Ti–6Al–4V powder at temperatures ≤400 °C. The upper limit to processing temperature was chosen to minimise the potential for contamination with gaseous constituents potentially harmful to properties of consolidated product. It has been demonstrated that the application of ECAE with imposed hydrostatic pressure permits consolidation to in excess of 96% relative density at temperatures in the range 100–400 °C, and in excess of 98% at 400 °C with applied back pressure ≥175 MPa. ECAE compaction at 20 °C (back pressure = 262 MPa) produced billet with 95.6% relative density, but minimal green strength. At an extrusion temperature of 400 °C, the relative density increased to 98.3%, for similar processing conditions, and the green strength increased to a maximum 750 MPa. The relative density of compacts produced at 400 °C increased from 96.8 to 98.6% with increase in applied back pressure from 20 to 480 MPa, while Vickers hardness increased from 360 to 412 HV. The key to the effective low-temperature compaction achieved is the severe shear deformation experienced during ECAE, combined with the superimposed hydrostatic pressure.     

Nanograin layer formation at crack initiation region for very‐high‐cycle fatigue of a Ti–6Al–4V alloy

The microstructural features and the fatigue propensities of interior crack initiation region for very‐high‐cycle fatigue (VHCF) of a Ti–6Al–4V alloy were investigated in this paper. Fatigue tests under different stress ratios of R = ?1, ?0.5, ?0.1, 0.1 and 0.5 were conducted by ultrasonic axial cycling. The observations by SEM showed that the crack initiation of VHCF presents a fish‐eye (FiE) morphology containing a rough area (RA), and the FiE and RA are regarded as the characteristic regions for crack initiation of VHCF. Further examinations by TEM revealed that a layer of nanograins exists in the RA for the case of R = ?1, while nanograins do not appear in the FiE outside RA for the case of R = ?1, and in the RA for the case of R = 0.5, which is explained by the Numerous Cyclic Pressing model. In addition, the estimations of the fatigue propensities for interior crack initiation stage of VHCF indicated that the fatigue life consumed by RA takes a dominant part of the total fatigue life and the related crack propagation rate is rather slow.     

Properties of Ti–6Al–4V non-stochastic lattice structures fabricated via electron beam melting

This paper addresses foams which are known as non-stochastic foams, lattice structures, or repeating open cell structure foams. The paper reports on preliminary research involving the design and fabrication of non-stochastic Ti–6Al–4V alloy structures using the electron beam melting (EBM) process. Non-stochastic structures of different cell sizes and densities were investigated. The structures were tested in compression and bending, and the results were compared to results from finite element analysis simulations. It was shown that the build angle and the build orientation affect the properties of the lattice structures. The average compressive strength of the lattice structures with a 10% relative density was 10 MPa, the flexural modulus was 200 MPa and the strength to density ration was 17. All the specimens were fabricated on the EBM A2 machine using a melt speed of 180 mm/s and a beam current of 2 mA. Future applications and FEA modeling were discussed in the paper.     

Effects of various environments on fatigue crack growth in Laser formed and IM Ti–6Al–4V alloys

The fatigue crack growth behaviors of Laser formed and ingot metallurgy (IM) Ti–6Al–4V alloys were studied in three environments: vacuum, air and 3.5% NaCl solution. Taking the Unified Fatigue Damage Approach, the fatigue crack growth data were analyzed with two intrinsic parameters, stress intensity amplitude ΔK and maximum stress intensity K_max, and their limiting values ΔK^* and . Fatigue crack growth rates da/dN were found increase with stress ratio R, highest in 3.5% NaCl solution, somewhat less in air and lowest in vacuum, and higher in IM alloy than in Laser formed one. In 3.5% NaCl solution, stress corrosion cracking (SCC) was superimposed on fatigue at R=0.9 for where K_max>K_ISCC, the threshold stress intensity for SCC. This and environment-assisted fatigue crack growth were evidenced by the deviation in fatigue crack growth trajectory (ΔK^* vs. curve) from the pure fatigue line where . Furthermore, the fractographic features, identified along the trajectory path, reflected the fatigue crack growth behaviors of both alloys in a given environment.     

Hot compressive deformation behavior and microstructure evolution of Ti–6Al–2Zr–1Mo–1V alloy at 1073 K

Hot compressive behaviors of Ti–6Al–2Zr–1Mo–1V alloy at 1073 K, as well as the evolution of microstructure during deformation process, were investigated in this paper. The results shows that flow stress increases up to a peak stress, then decease with increasing strain, and forms a stable stage at last. The grain size also shows an decrease at first and increase after a minimum value. Dislocations are observed to produce at the interface of α/β phase, and the phase interface and dislocation circle play an important role in impeding the movement of dislocation. As strain increase, micro-deformation bands with high-density dislocation are founded, and dynamic recrystallization occurs.     

Role of hydrogen environment induced hydrogen embrittlement of Ti–8Al–1Mo–2V alloy

Structural analysis by mean of metallographic, SEM fractographic and TEM replica technique including acoustics-emission studies have been carried out on Ti–8Al–1Mo–2V alloy specimen tested at room temperature in gaseous hydrogen environment. The result provided evidences of the presence of face centred cubic titanium hydride at the fracture surfaces, with discontinuous nature of crack propagation. The present work confirmed that an essentially continuous path of β phase is necessary for the occurrence of slow crack growth in gaseous hydrogen. Metallographic and fractographic observation leave little doubt that cracks propagates along the α–β interface rather than through stable α phase.     

Electrochemical corrosion properties of Ti–6Al–4V implant alloy in the biological environment

The electrochemical corrosion behavior of Ti–6Al–4V implant alloy was investigated in three biological solutions, i.e. urine, serum and joint fluid. The corrosion properties of Ti–6Al–4V implant alloys were examined by using electrochemical techniques, such as the potentiodynamic method, cyclic voltammetry, electrochemical impedance spectroscopy (EIS). The electrochemical corrosion characteristics of Ti–6Al–4V implant alloys in three biological solutions were measured in terms of the corrosion potential (E_corr), the corrosion current density (i_corr), and ac polarization resistance (R_p). The corrosion kinetic parameters were calculated from both the Tafel plot analyses and EIS analyses. The dependence of impedance versus potentials was studied at 37 °C at various offset potentials in three biological solutions. The ac circuit model for Ti–6Al–4V implant alloy at corrosion interface in biological solution was proposed, which was based on a simple Randles equivalent circuit. It was found that the Ti–6Al–4V implant alloy in three biological solutions showed a characteristic of a capacitive behavior. The experimental results of Tafel plot analyses were found in good agreement with that of EIS analyses.     

Study of high temperature mechanical behavior of the thermally oxidized Ti‐6Al‐4V alloy

The influence of oxidation of a Ti‐6Al‐4V alloy at 800 °C on its tensile properties at 600 °C has been studied. Specimens of this alloy were oxidized at 800 °C for 0.5, 1, 5, 10, 20 and 40 h. Tensile tests at 600 °C were carried out and the fracture surfaces were also examined. Oxidation of the specimens resulted in the formation of an oxide layer that spalled and another oxide layer that adhered to the substrate. Oxide formation increased with increase in duration of oxidation. In this investigation, density curves of the oxide layer as a function of duration of oxidation at 800 °C were used to identify a parabolic oxide growth law. The results of this study revealed coherence between the experimental data and calculations based on the Pilling‐Bedworth law. The mechanical strength of the Ti‐6Al‐4V alloy did not vary significantly with oxidation, but reduction in cross sectional area with increase in oxide layer thickness, as well as the slope of the stress‐strain curve decreased beyond the ultimate tensile strength. Fracture of the tensile tested specimens was predominantly ductile with microcavities. At certain regions of the oxide layer, brittle fracture with radial cracks was observed indicating intergranular fracture.     

Probabilistic modeling of fatigue crack growth in Ti–6Al–4V

This paper proposes an approximate approach to efficient estimation of some variabilities caused by the material microstructural inhomogeneities. The approach is based on the results of a combined experimental and analytical study of the probabilistic nature of fatigue crack growth in Ti–6Al–4V. A simplified experimental fracture mechanics framework is presented for the determination of statistical fatigue crack growth parameters from two fatigue tests. The experimental studies suggest that the variabilities in long fatigue crack growth rate data and the Paris coefficient are well described by the log-normal distributions. The variabilities in the Paris exponent are also shown to be well characterized by a normal distribution. The measured statistical distributions are incorporated into a probabilistic fracture mechanics framework for the estimation of material reliability. The implications of the results are discussed for the probabilistic analysis of fatigue crack growth.     

Al–Si coating fused by Al + Si powders formed on Ti–6Al–4V alloy and its oxidation resistance

An Al–Si coating was successfully produced by means of the low oxygen pressure fusing technology for improving the oxidation resistance of Ti–6Al–4V alloy. The Al and Si concentration in coating and coating thickness could be controlled by adjusting powder mixing ratio and changing the technical parameters (fusing temperature and time), respectively. At 1273 K, the weight gain of the Al–20Si coating increased with prolonging fusing time and its equation could be described as Δm² = 3.62t. After 105 h oxidation, the oxidation rate of the Al–20Si coated specimen with fusing time 100 min was about two to four times than that of the Al–10Si coated specimen with fusing time 60 min.     

The tensile deformation behavior of Ti–3Al–4.5V–5Mo titanium alloy

The tensile deformation behavior of Ti–3Al–4.5V–5Mo titanium alloy was studied. The results show that there are obvious yield points on true stress–true strain curves of annealing structures, then a stress drop occurs. The curves show linear work-softening after yielding at annealing temperature of 720–780 °C and linear work-hardening at annealing temperature of 800–840 °C. Elastic energy stored in the α-phase is dramatically released after plastic deformation of the β-phase, which leads to the stress drop.     

Investigation into effects of re-shot-peening on fretting fatigue behavior of Ti–6Al–4V

The effects of re-shot-peening treatment on fretting fatigue life/strength and the recovery of residual stress of the initially shot-peened Ti–6Al–4V were investigated at room and elevated temperatures. After subjecting to fretting fatigue up to about 40% of the total expected life of the initially shot-peened Ti–6Al–4V or to thermal exposure to 370 °C only, residual stress relaxed in the range of 20–50% of its value before fretting fatigue. The magnitude of stress relaxation depended upon the applied load level and test temperature. Re-shot-peening successfully recovered the relaxed residual stress up to the same level as obtained after the initial shot-peening. Further, fretting fatigue life after re-shot-peening, excluding pre-re-shot-peening fatigue life, was very close to that of the initially shot-peened specimen at a given stress level and test temperature. It thus appears that re-shot-peening nullified the effect of fretting fatigue damage after the initial shot-peening.